fuel gauge flips from dead empty to totally full

2005 GMC SERRIA 1500 4x4 NEW 5.3 vortex with RV cam. Fuel gauge stopped working so I replaced the fuel pump in the tank, still not working. Either all the way empty of full, never anything in-between. Thought I might have got a fuel pump so took it back exchanged for another new one. This time I looked up the specs in AllData and verified the ohms where correct on the new fuel pump. They specs were spot on matching. So I installed the 2nd pump still same thing, all the way full or empty. Next I thought it had to be the instrument cluster. Sent it in for a total rebuild. I told the tech what was goin on and asked him to bench test the old fuel gauge before tearing it down. He reported back saying the fuel gauge was working perfectly. So, I will be driving down the road and out of nowhere it will flip from full to empty and light comes on as well. It might stay in the empty position for one or two days maybe even a week and then will flip back to full and stay for awhile. Twice I have removed and cleaned the three chassis ground wires just under the drivers door. I have installed a new PCM & BCM as well. I don't know if these next two items
are related but I think its smart to include everything. So, maybe once or twice a month the odometer will read Service 4x4 however in works fine. And this next one is a real pain. 95% of the time when I get in to start it it will just crank and crank. Turn the key back to off and restart and boom starts within half a second. I drove myself nuts on this a few months ago and just threw my hands up. BUT I've had a good breather and I'm ready to WIN!!!
Parts to date: 2 fuel pumps, newly rebuilt instrument cluster, new PCM, new BCM
NO CODES
Don't have a fancy scanner but pretty sharp with a Fluke meter.

Just found this post where someone else was having the same problem. Has to crank it twice to start. See below, Thanks

2005 GMC Sierra 1500 188,000 mi, Visitor
My 2005 GMC Sierra 1500 fuel gauge started reading empty all of a sudden and then it would go back to normal and then when I stop at a light or somewhere, it would read empty again. On and off. Up and down is what going on with the fuel gauge. Then the engine light comes on right after this started happening. Now what!! Also it takes turning the engine over twice to get it to start when before all this started, it only took one try. Before this happen, I haven't had any problems with this truck. Does anybody know what the permanent fix for this problem is?

19taylor57, I'd say lets attack one problem at a time. If we happen to find the no start and the fuel gauge are related along the way, great. If not, you can go after the other problem.

The no start sounds like it's the symptom that's easier to reproduce? So I say lets start there. You say you don't have a fancy scanner, and that's fine. We can work with that.

Start with the basics while the no start is occurring. Spark, fuel, compression. Compression isn't likely a problem, but spark and fuel are definitely suspect. Have someone else behind the wheel ready for that first starting attempt while you're underhood. Take your pick of spark, fuel injection pulse or fuel pressure.

Does the engine have spark/fuel/pulse during the no start? Yes? Move on down the list. No? Go down that path and find out why. I'd start with a fuel pressure gauge on the rail, but that's just my first best guess. I know you just put two fuel pumps in it. Doesn't matter. Fuel pressure isn't good until you've verified it.

Paul has some fantastic videos for checking injection pulse and spark: